1 Answer
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The information below is from a developer website, it is the closest I could find to a definitive answer.

From website:

If you're using API Level 7 or lower, use getExternalStorageDirectory() to open a File that represents the root of the external storage, then save your shared files in one of the following directories:

* Music/ - Media scanner classifies all media found here as user music.
* Podcasts/ - Media scanner classifies all media found here as a podcast.
* Ringtones/ - Media scanner classifies all media found here as a ringtone.
* Alarms/ - Media scanner classifies all media found here as an alarm sound.
* Notifications/ - Media scanner classifies all media found here as a notification sound.
* Pictures/ - All photos (excluding those taken with the camera).
* Movies/ - All movies (excluding those taken with the camcorder).
* Download/ - Miscellaneous downloads.

i don't think you need to put the music in a specific folder, and that's the same thing for the wallpaper. When your phone starts (and sometimes), you can see something like "Indexing media" on the status bar. That is when Android crawl the SD card to find out this kind of file.
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RobSep 28 '10 at 13:03

1

I didn't help develop the Android OS, so I can't talk one way or the other on this, but I that link I provided is from the people that did write Android. My guess is that certain apps will look in multiple places for file types if told to do so, but by default that is the list.
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WebsSep 28 '10 at 20:57

This answer is very useful if you want to copy files from your pc onto the card and have them show up in the Videos gallery, for example: Creating a Movies directory and copying .mp4s into it worked like a charm.
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npdotyFeb 22 '11 at 18:58